Molten metal surface protective covers of this type have been known which are described in Japanese Patent Application Laid-Open No. 2-41741 and Japanese Patent Application Laid-Open No. 2-220740. In JP laid-open No. 2-41741, a central insulating member is provided which contacts the surface of a middle portion of the molten metal from above in a molten metal reservoir defined between a pair of rolls (an example of moving mold walls). The central insulating member is provided with side insulating members which cover the space above locations adjacent the points of contact between the roll surfaces and the molten metal. Immersion walls concentric with respective roll surfaces project downwardly from side edges of the central insulating member which are formed parallel to the rolls, the immersion walls being spaced a predetermined distance (on the order of 15 to 40 mm) from the roll surfaces.
In JP Laid-Open No. 2-220740, an insulating member floating on the molten metal surface is provided in a molten metal reservoir formed between a pair of rolls. Immersion walls concentric with respective roll surfaces project downwardly from side edges of the insulating member which are formed parallel to the rolls, the immersion walls being spaced a predetermined distance from the roll surfaces.
According to such known arrangement, the molten metal surface is covered with an insulating member and is therefore prevented from becoming oxidized and suffering a decrease in its temperature. The immersion walls at opposite side of the insulating member permit fresh molten metal to flow into the spaces between respective moving mold walls and respective immersion walls. As a consequence, the surface layer of a cast piece shell produced on the surface of each moving mold wall becomes remelted. Thus, the cast piece is prevented from becoming unevenly thick.
However, with such known arrangement, wherein the spacing between each immersion wall and the roll closer thereto is relatively large, flow stagnation may occur on the molten metal surface and, once such stagnation occurs, slag present in the molten metal tends to float up to the surface and gradually accumulate on the molten metal surface to grow into a large mass. Such a grown slag mass may be drawn toward the direction of roll rotation until it forms a part of the cast piece. This slag has poor heat transfer behavior, which results in insufficient cooling of any portion of the cast piece in which such slag is present, eventually leading to cast piece cracking.